We started making this game a few months after Panty Party was released. We usually started with 1 or 2 people having a playable prototype done, and after that more people came in to help on art and music and such things. And will eventually turn into a group of around 10 people.
We made games with Unity engine so Unity is a must buy thing. xD And depending on the thing you are working on, you need extra tools to make 3D art, 2D art, music and sound effect.